The composition of basal medium (g L1) used in this study was: NaNO3 (7); K2HPO4 (1); KH2PO4 (0.5); KCl (0.1); MgSO4 7H2O (0.5); CaCl2 (0.01); FeSO4 7H2O (0.01) and yeast extract, (0.1). The medium was supplemented with 0.05 mL of trace elements solution of the following composition (g L1): H3BO3 (0.25); CuSO4 5H2O (0.5); MnSO4 H2O (0.5); MoNa2O4 H2O (0.06) and ZnSO4 7H2O (0.7) (Mercade´ et al., 1996). The pH was adjusted to 7.5 and the media was autoclaved. Diesel oil was sterilized three times by filtration through 0.2 mm. membrane filters.